Representative Dana Rohrabacher, the Republican incumbent, is known for outrageous statements, pro-Russia views and solid support for President Trump; his deeply Republican district has re-elected him for three decades. But Democrats see him as the most vulnerable Republican in the state, and he is in a tossup contest with Harley Rouda, a real estate executive and former Republican who switched parties. “President Trump has changed everything,” Mr. Rouda has said.